In response to a growing need to improve the efficiency of radio communications systems, a Digital Mobile Radio (DMR) communications standard establishes protocols and other system requirements for communications on private mobile radios. Any DMR standards or specifications referred to herein may be obtained by contacting European Telecommunications Standard Institute (ETSI) at ETSI Secretariat, 650, Route des Lucioles, 06921 Sophia-Antipolis Cedex, FRANCE. A radio, as used herein, can be any mobile and/or fixed end equipment that is used to obtain DMR services. The DMR standard supports simultaneous and independent calls on a single radio channel. In particular, according to the standard, a 12.5 Kilohertz (kHz) bandwidth radio channel is divided into two alternating timeslots. Each timeslot acts as a separate communications path (sometimes also referred to as a channel or logical channel), which is different from a radio channel (physical channel) for two or more radios. Each radio operating on the radio channel communicates on an assigned timeslot according to, for example, a Time Division Multiple Access (TDMA) protocol.
To avoid interference between radios transmitting on the same physical channel (i.e., the same radio frequency (RF) channel) but different time slots, each radio should be aware of a common “channel timing” or “timeslot structure” for the physical channel. As used herein, channel timing is defined as a timing reference that defines boundaries for one or more of the time slots that make up the physical channel. The ETSI-DMR standard currently describes a means for an infrastructure device, such as a repeater, to provide common channel timing for radios operating in repeater mode. Therefore, when radios are operating in repeater mode, two different non-interfering calls can be supported on two TDMA time slots of a radio channel.
The ETSI-DMR standard also allows for radios to operate in talkaround or direct mode. In the talkaround or direct mode any radio can communicate with one or more other radios without the need for any additional infrastructure equipment (e.g. base stations or repeaters). Radios may operate in talkaround mode (in a system where the radios primarily use a repeater and occasionally communicate without a repeater) or in direct mode (in a system where the radios exclusively communicate without a repeater). In the current standard, when radios operate in talkaround or direct mode one timeslot is used and the other is left unused, which is spectrally inefficient because the available bandwidth is only being used for one half of the available time. A current solution to leaving one timeslot unused provides for using both timeslots (also known as 2:1 TDMA) when operating in direct mode or talkaround mode (herein referred to as operating in 6.25e TDMA Direct Mode or simply as operating in direct mode).
To minimize channel traffic associated with radios requesting the channel timing, one radio operating on the radio channel (in either timeslot) is selected as a channel timing leader. The channel timing leader intermittently transmits a timing signal in at least one channel timing (CT) message to other radios operating on the radio channel. The other radios operating on the radio channel adjust their transmission clocks based on the timing signal transmitted from the timing leader. The radios capable of receiving the timing signal transmitted from the timing leader also diffuse or propagate the timing signal to other radios operating on the RF channel which would cause clock adjustments triggered by the leader or other radios. The 6.25e TDMA Direct Mode feature requires that all direct mode radios monitor a predefined radio channel for the CT message and cooperatively define and maintain (over time) the TDMA channel structure to be used with all transmissions on the radio channel.
However, radios may also implement a scan feature. The scan feature allows the radio to constantly check for activity on multiple channels in the radio's unique scan list by monitoring a set of specific channels (frequencies and timeslots) according to a predetermined sequence, when scanning. Radios implementing the scan feature are therefore unable to continuously monitor the predefined radio channel for the CT message to participate in the management of TDMA channel timing.
Accordingly, there is a need for a method and apparatus for allowing radios operating in direct mode to both track channel timing and support channel scanning.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.
The apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.